This disclosure relates to the field of well drilling. More specifically, the disclosure relates to pumps used to maintain fluid pressure in a well during drilling operations.
U.S. Pat. No. 6,904,981 issued to van Riet describes a well pressure control system that may be used in the construction of subsurface wells. The function of the well pressure control system disclosed in the van Riet '981 patent is to maintain fluid pressure in the well higher than the hydrostatic pressure exerted by a column of fluid of a selected density at any true vertical depth in the well. Such fluid pressure is maintained by a controllable orifice choke disposed in a fluid outlet or discharge conduit from the well, where the well is closed to fluid flow other than through a drill string disposed in the well and the fluid outlet or discharge conduit. The controllable orifice choke provides a backpressure to the well resulting from restriction of fluid flow out of the well when fluid is pumped into the well through the drill string. During times when fluid is not pumped into the drill string, a backpressure pump or flow diverted from drilling rig mud pumps to the fluid outlet or fluid discharge conduit may be used to maintain a selected backpressure, and consequent selected fluid pressure in the well. Maintaining fluid pressure may require pumping additional fluid into the well using a backpressure pump or diverted flow from the drilling rig mud pumps in particular during “tripping” operations, where the drill string is withdrawn from the well. Withdrawal of the drill string from the well reduces the amount of well fluid displaced by the drill string, thus enabling the well fluid pressure to decrease; thus additional fluid may be pumped into the well to maintain the fluid pressure. Separate backpressure pumps may be preferable in some circumstances because they may be more accurately controlled than the drilling rig mud pumps. There is a need for improved backpressure pumps to enable more precise well pressure control.